The present invention relates to high pressure, plasma discharge devices. In particular, the present invention pertains to the stability of high pressure plasma columns within such devices conferred by the mechanical rotation of their envelopes. Also, the present invention relates to methods and apparatuses for forming stable plasma columns at high pressure in both open-ended and sealed discharge containment envelopes so as to produce plasma torches, waste reprocessing devices, ultraviolet, visible, infrared, and x-ray light sources, or high intensity illumination.
Over the years, many devices have been evolved which attempt to generate, control and use high pressure plasmas for various applications. Each of these devices has certain disadvantageous limitations. The embodiments of the present invention overcome many of the disadvantages of existing technologies by utilizing rotation of the containment envelope to establish a nearly perfect, rigid rotor flow of the gases within. The artificial gravity associated with this rotational flow acts to both center and confine a bubble of plasma and hot gases.
In the context of the present patent document a plasma is a vapor (usually created from a plasma forming fill in gaseous phase) which includes both neutral particles and charged particles, the latter consisting of electrons and ions. The ions, in turn, may be a combination of atomic ions, charged radicals and/or molecular ions, in which the balance among these different species is dependent on temperature, pressure and the nature of the plasma forming fill. By carefully controlling the environmental conditions in and around the plasma, a plasma column can be formed which is physically isolated from the material boundary of the confinement envelope by an intervening layer of neutral gas. Thus, the plasma column may occupy a cylindrical volume that is smaller than the volume enclosed by the containment envelope.
Since the early 60s, high pressure rf (radio frequency) heated discharges have relied on the use of a xe2x80x9cswirl gasxe2x80x9d to center the plasma columns produced in such electrodeless devices and to prevent hot plasma from contacting the walls of the containment envelope. Because of the circulatory flow of the injected swirl gases, the term xe2x80x9cvortex stabilizationxe2x80x9d is generally used to characterize this technique. The swirl gas method of vortex stabilization has been applied to a broad range of rf devices. See, for example, Boulos, M. I., in xe2x80x9cThe Inductively Coupled Radio Frequency Plasma,xe2x80x9d High Temp. Material Processes, 1, 17 (1997) or Reed, T. B., J. Appl. Phys., 32, 821 (1961) both of which are hereby incorporated by reference. The swirl gas technique makes it possible to establish plasma columns within containment envelopes at gas pressures from a fraction of atmospheric pressure to many times atmospheric pressure. The plasma columns need not be straight. With the aid of vortex stabilization, a high pressure, rf-heated toroidal discharge has been achieved. See, for example, A. Okin et al., in xe2x80x9cGeneration of Toroidal Plasmaxe2x80x94Atmospheric Ar Gas-Insulated Plasma Source with Quartz and Metallic Discharge Tubes,xe2x80x9d Proc. Symp on Plasma Science for Materials, ISSN 0919-7621 (1993) which is hereby incorporated by reference.
In addition to electrodeless discharges heated with rf power, the swirl gas method of vortex stabilization has been applied in electrodeless, high pressure torches powered with microwaves. (e.g., U.S. Pat. No. 5,671,045).
Although the plasma columns formed by the swirl gas technique of vortex stabilization are approximately centered within their containment envelopes and stationary in a gross sense, they are not quiescent on the microscopic level. Viscous drag on the stationary wall of the containment envelope results in a sheared flow of the gas which generates turbulence, giving rise to enhanced thermal transport of energy from the hot plasma column to the cooler envelope walls. Such a degradation in the insulating properties of the annular sheath of cooler gas surrounding the plasma column necessitates an increased power to sustain the plasma discharge. Additionally, turbulence causes mixing of the gas within the sheath and between the gas in the plasma column and that in the sheath. As a practical matter, when such a technique of vortex stabilization is used, the turbulence so generated mainfests itself as excess noise in the light emission of the plasma column, which can degrade the accuracy of certain spectroscopic measurements. As a further practical matter, the tip of the plasma plume in vortex stabilized torches is not perfectly stable, demonstrating the characteristic of xe2x80x9cwanderingxe2x80x9d or xe2x80x9cflickeringxe2x80x9d with respect to the center axis of the torch. Therefore, such torches are unsuitable for applications in which precise positioning is required.
The concept of rotating a containment envelope has been used in xe2x80x9csulfur bulbxe2x80x9d lamp technologies (e.g., Pat. Nos. 4,902,935 and 5,404,076) to minimize variations in the surface temperature of a spherical bulb (hot spots, caused by locally high electric fields occurring in resonant microwave cavities or in coaxial termination fixtures) and to make the spatial distribution of visible light emission from these bulbs more uniform. The patents referenced above deal only with sealed bulbs powered by microwaves for application to high intensity lighting. To eliminate hot spots these patents state that the axis of rotation should be oriented in a certain angular range with respect to the electric field direction in the resonant microwave cavity.
Therefore, what is needed is a method and apparatus for producing a stable, high pressure plasma discharge that can be sustained with a minimum of power. Also, there is a need for creating stable plasma columns which are both long and straight. Also, there is a need for methods and apparatus for generating a stable plasma column inside a containment envelope whereby the effects of shear flow-generated turbulence and buoyancy-driven radial convection are substantially reduced in the gas outside the radius of the plasma column. There is also a need for methods and apparatus for forming a plasma torch having a plasma plume, or flame, which maintains a stable position centered on an axis of rotation of a containment envelope.
The present invention provides methods and apparatus for forming a stable equilibrium for a high pressure plasma column in a stable equilibrium.
In various embodiments a stable, high pressure plasma column is produced by an apparatus comprising i) a containment envelope having a plasma-forming fill inside, ii) means for rotating the containment envelope, iii) means for initiating a discharge (that is, achieving xe2x80x9cbreakdownxe2x80x9d or xe2x80x9cignitionxe2x80x9d) in the plasma-forming fill, and iv) means for heating and sustaining the resulting plasma in a steady state or pulsed manner. Furthermore, particular embodiments permit adjusting the diameter of a cylindrical surface enclosing the plasma column by varying the rate of rotation of the containment envelope.
Ignition of the high pressure discharge inside the rotating containment envelope may be accomplished by means of electrical, electromagnetic (such as radio frequency waves, microwaves, or light waves) or chemical sources of energy. These same sources of energy, individually or in combination with one another, may be used to heat and sustain the resulting plasma in a steady state or pulsed manner. The means of ignition and sustainment may utilize the same source of energy, or different ones.
Various embodiments provide methods and apparatus for rotating said envelope so as to bring the vapors inside the containment envelope into co-rotation with the spinning envelope. Under such conditions of rigid rotor flow, a radially outward directed, artificial gravity is established which forces more dense gases (colder gases) outward toward the wall of the containment envelope and less dense vapors (plasma and neutral gases that have been heated) inward toward the axis of rotation. Buoyancy-driven radial convection of the contained vapors is suppressed in an annular region outside the cylindrical surface at which the magnitude of the artificial gravity produced by rotation just equals the magnitude of the radially directed component of earth""s gravity. The annular region in which radial convection is suppressed is hereinafter referred to as the sheath region. Also, turbulence caused by sheared circulatory flow is absent in this sheath region. The excellent insulation properties of this quiescent gas sheath makes it possible to sustain an interior discharge column with a minimum of power. However, the artificial gravitational force produced by rotation becomes increasingly weak with decreasing radius, falling to zero on the rotational axis. Inside the radius at which the constant downward pull of earth""s gravity dominates in magnitude over the centrifugal force induced by rotation, buoyancy-driven radial convection can not be suppressed. Gas inside this radius mixes and rigid rotor flow can not be sustained.
Further embodiments contemplate both sealed and open-ended containment envelopes. In the open-ended ones, a forced axial flow of gas may be produced by injecting gas at positive pressure parallel to the rotation axis; or a natural, buoyancy-driven axial flow, by inclining the rotation axis of the envelope. Axial flow, forced or natural, can be used to extend the plasma column out one end of the open-ended envelope to form a plasma plume or xe2x80x9cflamexe2x80x9d of a plasma torch.
Additionally, the principles illustrated by the embodiments described herein contemplate methods and devices for making very long plasma columns.
Other features of the present invention are disclosed or made apparent in the section entitled xe2x80x9cDETAILED DESCRIPTION OF THE INVENTIONxe2x80x9d.